For example, suppose you have a two minute long video consisting of two distinct halves. The first half is a very high-motion scene lasting 60 seconds which, in isolation, requires about 2500kbps in order to look decent. Immediately following it is a much less demanding 60-second scene that looks good at 300kbps. Suppose you ask for 1400kbps on the theory that this is enough to accomodate both scenes. Single pass ratecontrol will make a couple of "mistakes" in such a case. First of all, it will target 1400kbps in both segments. The first segment may end up heavily overquantized, causing it to look unacceptably and unreasonably blocky. The second segment will be heavily underquantized; it may look perfect, but the bitrate cost of that perfection will be completely unreasonable.
Multipass ratecontrol can offer huge advantages over a single pass. Using the statistics gathered from the first pass encode, the encoder can estimate, with reasonable accuracy, the "cost" (in bits) of encoding any given frame, at any given quantizer. This allows for a much more rational, better planned allocation of bits between the expensive (high-motion) and cheap (low-motion) scenes.
for example, adding subq=1:frameref=1 to the first pass x264encopts.
Multipass ratecontrol can offer huge advantages over a single pass. Using the statistics gathered from the first pass encode, the encoder can estimate, with reasonable accuracy, the "cost" (in bits) of encoding any given frame, at any given quantizer. This allows for a much more rational, better planned allocation of bits between the expensive (high-motion) and cheap (low-motion) scenes.
for example, adding subq=1:frameref=1 to the first pass x264encopts.
